Switching Power Supply With Overvoltage Protection And Overvoltage Protection Method Thereof

ABSTRACT

A switching power supply with overvoltage protection includes a soft start circuit, a rectifying circuit, a filter capacitor, a main supply, an auxiliary supply and a monitoring circuit. When the input voltage is higher than the predetermined protection voltage, a first electric control switch is turned off, and the main power circuit of the switching power supply is shut off. At the moment, the actuation and release of the second electric control switch is controlled so as to control the input power supply to intermittently charge the power supply module such that the output voltage downstream of the rectifying circuit inside the power supply is controlled in a safe range. For instance, the second electric control is turned on when the output voltage is lower than a certain value, and is turned off when the output voltage is higher than a certain value. Thus, it is ensured that the bus line voltage inside the power supply is controlled within a safe range, to ensure the safety of the devices inside the power supply; and at the same time to ensure the safety of the inner devices of the switching power supply, such that the auxiliary power supply and the control circuit can work normally.

TECHNICAL FIELD

The present invention relates to a switching power supply withovervoltage protection and overvoltage protection method thereof.

DESCRIPTION OF THE RELATED ART

In a switching power supply module, topology of circuit as shown in FIG.1 is usually employed, comprising a soft start circuit, a rectifyingcircuit, a filter circuit (generally consisting of a filter capacitor),a main power supply (consisting of a main DC/DC converter and its outputfilter), an auxiliary power supply and a monitoring circuit. A powerfactor correction circuit, namely PFC circuit is provided between therectifying circuit and the main power supply in some of the switchingpower supplies.

If the voltage of the input power supply is too high, then someprotective measures have to be employed for preventing damage to thepower supply module. Common protective measure is to shut off the maincircuit (PFC and the DC/DC converter) of the power supply, to stop thepower output, and to switch off the input soft start relay, so as toensure the decreasing of inrush after the input power supply returns toa normal state. The input power supply continues to supply a current tothe filter circuit through a soft start resistor and the rectifyingcircuit when the relay is in an OFF state, so as to insure that theauxiliary power supply and the monitoring circuit continue to functionnormally. At the moment, the input voltage is decreased to a certainextent due to the voltage dividing function of the soft start resistor,so as to obtain certain protective effect.

Aforesaid protection only functions in a certain range of voltage of theinput power supply. Therefore, in the situation of overvoltage, thevoltage of the input power supply can not be too high, otherwiseinsufficient withstand voltage will result in damage. However, there maybe a very high input voltage, e. g. when the input phase voltage isconnected to a line voltage by accident, the input voltage will be ashigh as 380 Vac, and the peak voltage will exceed 500 Vdc, and manydevices in the power supply module have insufficient withstand voltageat the moment. Therefore, under a desired overvoltage protection, theexcessive voltage should be disconnected from the successive stage,namely overvoltage disconnecting rather than only voltage division.

However, in the switching power supply of prior art, the module can notachieve a overvoltage disconnecting protection when the AC voltage ofthe module is too high, because the operation of the auxiliary powersupply stops after being disconnected from the AC voltage, and there isno power supply for maintaining the disconnecting device. Thus, anadditional protective device is required, for example, by adding anisolation device such as an AC contactor on the system, and theisolation of the power network is achieved by controlling of connectingand disconnecting of the isolation device. However, there exist thefollowing problems that: 1) an additional device is required, which maycause the increase of the cost; 2) the whole system can not worknormally when the additional device malfunctions.

Apparently, method such as tripping may be employed to completely shutoff the power supply, and protection to the successive devices may beachieved. However, by doing so, the module can not return to a normaloperation state after the input voltage returns to a normal state, thuscan not meet the requirement.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a switching powersupply with overvoltage protection and overvoltage protection methodthereof, to solve the protection problems of the module of the prior artunder an overvoltage, and to return to a normal work state in time afterthe overvoltage.

To achieve the aforesaid object, the present invention provides aswitching power supply with overvoltage protection and overvoltageprotection method thereof.

The switching power supply with overvoltage protection comprises a softstart circuit, a rectifying circuit, a filter capacitor, a main powersupply, an auxiliary power supply and a monitoring circuit; the softstart circuit comprises a soft start resistor and a first electriccontrol switch positioned in a branch in parallel with the soft startresistor; after AC current is inputted into the rectifying circuit forrectifying and into the filter capacitor for filtering via the softstart circuit, the resulted DC current is outputted to the main powersupply and the auxiliary power supply respectively; the auxiliary powersupply supplies DC voltage to the monitoring circuit, while themonitoring circuit monitors the input or/and output voltage of theswitching power supply, and controls the first electric control switchof the soft start circuit; the present invention is characterized inthat a second electric control switch is provided, wherein the secondelectric control switch is connected in series with the soft startresistor, the control end of the second electric control switch isconnected with the monitoring circuit, the monitoring circuit controlsthe ON or OFF state of the second electric control switch.

The aforesaid solution may have the following modification orimprovement:

The present invention may further comprise a second impedance connectedin parallel with the second electric switch.

The present invention may further comprise a second impedance and athird impedance, the third impedance and the second electric controlswitch are connected in series, and then are connected with the secondimpedance in parallel.

The present invention may further comprise a second auxiliary powersupply, the input end of which is connected with a battery, and theoutput end of which is connected with the monitoring circuit.

The first and second electric control switches are relays.

The overvoltage protection method for a switching power supply comprisesthe following steps: S1: a monitoring circuit monitors the input voltageor/and output voltage of a switching power supply; S2: if AC input is inthe allowable range, then the open and close states of the firstelectric control switch of normal open type and the second electriccontrol switch of normal close type are controlled such that AC chargesthe filter capacitor through the soft start resistor and the secondelectric control switch, after the completion of the soft start, thefirst electric control switch is controlled to be turned on and themodule enters a normal operating state; S3: when the AC input has anovervoltage, the voltage of the filter capacitor rises, if a voltagehigher than the predetermined voltage threshold on the filteringcapacitor is detected by the monitoring circuit, the control circuitcuts off the first and second electric control switches K1, K2, and theenergy stored in the filter capacitor maintains the OFF state of thesecond electric control switch; S4: when the energy stored in the filtercapacitor is consumed and its voltage is lowered to a certain threshold,the second electric control switch returns to the ON state such that ACcan charge the filter capacitor through the soft start resistor and thesecond electric control switch, therefore, the voltage of the filteringcapacitor rises gradually; S5: if the voltage of the filter capacitorexceeds the allowable value again, the second electric control switchwill be cut off again and the aforesaid steps S3, S4 repeat; S6: untilthe voltage of AC input decreases to be in the normal range, the statesof the first and second electric control switches K1 and K2 arecontrolled such that the module enters into a normal operating state.

The aforesaid solution may have the following modification orimprovement:

A second voltage threshold is set in advance in the monitoring device,such that the second electric control switch is controlled by themonitoring circuit to return to the ON state when it is detected thatthe voltage is lower than the threshold.

The time of turning on or turning off of the first and second electriccontrol switches is set, at the time the input voltage being lower thanthe bus line voltage, such that there is no current when the relay turnson or cuts off.

The advantages of the aforesaid technical solutions lie in that when theinput voltage is higher than the predetermined protection voltage, afirst electric control switch is turned off, and the main power circuitof the switching power supply is shut off. At the moment, the onlybranch that can supply electric energy to the successive stages is thesoft start resistor branch. Since the second electric control switch ispositioned in this branch, or positioned in the DC bus line afterrectification, the actuation and release of the second electric controlswitch is controlled to control the input power supply to intermittentlycharge the power supply module, such that the output voltage after therectifying circuit inside the power supply is controlled in a certainrange, e.g. the second electric control is turned on when the outputvoltage is lower than a certain value (the value may be zero), and isturned off when the output voltage is higher than a certain value. Thus,it is ensured that the bus line voltage inside the power supply iscontrolled within a safe range, to ensure the safety of the devicesinside the power supply; and at the same time to ensure the safety ofthe inner devices of the switching power supply, such that the auxiliarypower supply and the control circuit can work normally. In other words,the auxiliary power supply and overvoltage protection is maintained byan alternating mode.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic view of the power supply module of aconventional communication device of prior art.

FIG. 2 shows a schematic view of the power supply module of acommunication device of the present invention.

FIG. 3 a shows a schematic view of the principle of a partial circuitaccording to embodiment 1 of the present invention (the main powersupply and its downstream output end are omitted).

FIG. 3 b shows a schematic view of the operating waveform IZ1 of a firstcontrol strategy of embodiment 1 of the present invention.

FIG. 3 c shows a schematic view of the operating waveform of the drivingsignal Q2 of a first control strategy of embodiment 1 of the presentinvention.

FIG. 3 d shows a schematic view of the input voltage waveform and thevoltage waveform of the driving coil of the auxiliary relay of anothercontrol strategy of embodiment 1 of the present invention.

FIG. 4 a shows a schematic view of the embodiment 2 of the presentinvention.

FIG. 4 b shows a schematic view of the embodiment 3 of the presentinvention.

FIG. 5 shows a schematic view of the embodiment 4 of the presentinvention.

FIG. 6 shows a schematic view of the embodiment 5 of the presentinvention.

FIG. 7 shows a schematic view of the embodiment 6 of the presentinvention.

FIG. 8 a shows a schematic view of a first implementation of the presentinvention.

FIG. 8 b shows the voltage-current waveform of the zero-voltageoperating of the relay of the first implementation of the presentinvention.

FIG. 8 c shows the release waveform of the relay.

FIG. 9 shows a schematic view of a second implementation of the presentinvention.

FIG. 10 shows a schematic view of a third implementation of the presentinvention.

FIG. 11 shows a schematic view of a fourth implementation of the presentinvention.

FIG. 12 shows a schematic view of a fifth implementation of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a general schematic view of the several embodiments of thepresent invention discussed below, wherein the second relay is shown indashed line since it has two alternative positions. As shown in FIG. 2,the switching power supply with overvoltage protection of the presentinvention comprises a soft start circuit 1, a rectifying circuit 2, afilter capacitor C1, a main power supply 3, an auxiliary power supply 4and a monitoring circuit 10, wherein the soft start circuit 1 comprisesa soft start resistor Z1 and a first electric control switch K1positioned in a parallel branch of the soft start resistor Z1; after ACcurrent is inputted into the rectifying circuit 2 for rectifying andinto the filter capacitor C1 for filtering via the soft start circuit 1,the resulted DC current is outputted to the main power supply 3 and theauxiliary power supply 4 respectively; the auxiliary power supply 4supplies DC voltage to the monitoring circuit 10, while the monitoringcircuit 10 monitors the input or/and output voltage of the switchingpower supply and controls the first electric control switch K1 of thesoft start circuit. The primary improvement of the present inventionlies in that a second electric control switch K2 is provided. The secondelectric control switch K2 is connected in series on the phase line onwhich the soft start resistor Z1 is disposed. The control end of thesecond electric control switch K2 is connected with the monitoringcircuit 10 which controls the ON and off state of the second electriccontrol switch K2.

The present invention will be further described in details below withreference to the accompanying drawings and the preferred embodiments.The first, second electric control switches K1, K2 are relays in theseembodiments, but are not limited to relays. Thyristor can also be used.

Embodiment 1

As shown in FIG. 3 a, the first and second electric control switches K1,K2 are relays in the present embodiment, wherein the second relay K2 andthe soft start resistor Z1 are connected in series, and then connectedwith the first relay K1 in parallel.

There are two kinds of control strategies as shown in the followingdepending on the step S4:

1) The first control strategy: The first electric control switch K1 isset to be in a normal open state, and the second electric control switchK2 is set to be in a normal close state in advance, such that when theenergy stored in the filter capacitor C1 is consumed and is insufficientto actuate the second electric control switch K2, the second electriccontrol switch K2 will be closed automatically, and return to an ONstate.

2) The second control strategy: A voltage threshold is set in themonitoring device 10 in advance. When it is detected that the voltage islower than the threshold, the second electric control switch K2 iscontrolled by the monitoring circuit 10 to return to an ON state.

The operating principle of the first kind of control strategy of thepresent embodiment is shown in FIGS. 3 b, 3 c.

The AC input of the module is converted to DC after being rectified andfiltered, then is outputted as respective operating voltage after beingfurther converted. To achieve the normal operation of the controlcircuit, this kind of module will convert the rectified DC into one ormore low power supply, namely the auxiliary power supply as shown in thedrawings, to maintain the operating of the control circuit. K1 is in anormal open state and K2 is in a normal close state, to insure that thecircuit can be started normally.

State 0: when the AC input is in the allowable range, AC charges thehigh voltage capacitor (namely the filter capacitor C1) through Z1, K2.After the completion of the soft start, K1 turns on and the moduleenters a normal operating state.

State 1: when the AC input is under an overvoltage, the voltage of C1rises. If voltage of C1 does not exceed the allowable voltage of thecapacitor, no action is performed; if a voltage of C1 higher than theallowable voltage of the capacitor is detected, the control circuit cutsoff K1, K2, and the energy stored in the capacitor C1 maintains the OFFstate of K2 while the normal open device K1 does not consume any energy.When the stored energy is used up or nearly used up, K2 returns to an ONstate, and the module supplies energy to the high voltage DC capacitorC1 again. Therefore, the voltage of C1 rises gradually. If voltage of C1exceeds the allowable voltage again, K2 will be cut off again and theaforesaid steps repeat; if the voltage of the capacitor C1 does notexceed the allowable voltage, K2 will not be cut off; if the voltage ofAC input decreases to be in the normal range, K2 is actuated and K1 isactuated in a delay mode, the module starts to a normal work.

Thus, failure of the module under high voltage is prevented by thealternating mode of K2. It is noticed that the actuation and release ofK2 is achieved by the nature character of charge and discharge of C1. Inthis way, since K2 will be switched automatically when the voltage islowered to a certain value, the monitoring circuit 10 does not need tomonitor the lowest value of voltage.

The aforesaid operating principle is only one of the control strategiesof the present embodiment, and actually there exist other controlstrategies. For example, the second control strategy may be employed forpreventing the voltage of the successive stages (e.g. the auxiliarypower supply) from being too low, the above mentioned second strategycan be employed. In particular, the lowest value of voltage is detectedby the monitoring circuit 10, and a threshold of voltage is set inadvance in the monitoring device 10, when it is detected that thevoltage is lower than the threshold, the second electric control switchK2 is controlled by the monitoring circuit 10 to return to ON state.FIG. 3 d is a schematic view of the control strategy, which will bediscussed in detail below.

It is appreciated that the overvoltage protection method comprises thefollowing steps by summarize the above operating principle:

S1: the monitoring circuit 10 monitors the input voltage or/and outputvoltage of the switching power supply; S2: if the AC input is in theallowable range, then the open and close state of the first and secondelectric control switch K1, K2 are controlled, such that AC charges thefilter capacitor C1 through soft start resistor Z1 and the secondelectric control switch K2, then after the completion of the soft start,K1 turns on and the module enters a normal operating state; S3: when theAC input has an overvoltage, the voltage of C1 rises, if a voltagehigher than the predetermined voltage threshold is detected on C1 by themonitoring circuit 10, the control circuit cuts off K1, K2, and theenergy stored in the filter capacitor C1 maintains the OFF state of thesecond electric control switch K2; S4: when the energy stored in thefilter capacitor C1 is used up and its voltage is lowered to a certainthreshold, the second electric control switch K2 returns to the ONstate, such that AC can charge the filter capacitor C1 through the softstart resistor Z1 and the second electric control switch K2, therefore,the voltage of C1 rises gradually; S5: if the voltage of the filtercapacitor C1 exceeds the allowable voltage again, the second electriccontrol switch K2 will be cut off again and the aforesaid steps S3, S4repeat; if the voltage of AC input decreases to be in the normal range,then the states of the first and second electric control switches K1, K2are controlled, such that the module enters into a normal operatingstate.

Embodiment 2

As shown in FIG. 4 a, the present embodiment is characterized by animpedance Z2 connected in parallel with K2. It is preferable that aresistor is adopted as Z2. In FIG. 4 a and figures thereafter, themonitoring circuit 10 is consisting of a logical decision circuit and adriving circuit.

The improved circuit has the same operating mode, and is characterizedin that AC input continues to supply energy to the high voltage C1through impedances Z1, Z2 when Z2 is in OFF state, such that themaintaining time of the auxiliary power supply, that is the duration ofOFF state of K2 is elongated. Thus, the power consumption of Z1 isreduced significantly.

In the case that the resistance of Z2 becomes infinite, embodiment 2 isthe same with embodiment 1.

Embodiment 3

As shown in FIG. 4 b, a third impedance Z3 connected in series with thesecond electric control switch K2 is provided compared with embodiment2, and serially connected Z3 and K2 are connected in parallel with thesecond impedance Z2. This is because that Z3 is needed for avoiding anarc when Z2 is a capacitor.

Embodiment 4

As shown in FIG. 5, double auxiliary power supplies are used in thepresent embodiment, that is, a second auxiliary power supply isprovided, the input end of which is connected with the battery, and theoutput end of which is connected with the monitoring circuit 10.

In the case of double auxiliary power supply, if the energy storagedevice such as battery is in a normal state, when AC has an overvoltage,the auxiliary power supply may continuously supply energy formaintaining the OFF state of K2.

When the AC input is under an overvoltage, the voltage of C1 rises. Ifvoltage of C1 does not exceed the allowable voltage of the capacitor, noaction is performed; if the voltage of C1 higher than the allowablevoltage of the capacitor is detected, the control circuit cuts off K1,K2, and the energy stored in the capacitor C1 maintains the OFF state ofK2 while the normal open device K1 does not consume any energy. Thebattery maintains the OFF state of K2 through the conversion of theauxiliary power supply. If the voltage of AC input decreases to be inthe normal range, K2 is actuated and K1 is actuated in a delay mode, themodule starts to work normally.

If the battery does not exist or is used up, then the overvoltageprotection is achieved in the above-mentioned manner.

Embodiment 5

As shown in FIG. 6, the position of K2 is changed in the presentembodiment, such that Z1 and K1 are connected in parallel, and then areconnected to Z2 in series. The circuit is characterized in that ACsupplies energy to the auxiliary power supply through Z1, Z2 and K2rectifying devices after K1 is shut off.

Embodiment 6

As shown in FIG. 7, the position of K2 is changed in the presentembodiment, however, K2 is positioned downstream of the rectifyingcircuit. Thus, the rectifying circuit is not under the overvoltageprotection.

The applications of the present invention in various situations aredescribed below with reference to FIGS. 8 a-12. The driving circuitportion (namely, Q1, Q2 etc.) of FIG. 3 a and some other detailedportions are not shown in these views. Actually, the driving circuitportion in FIG. 3 also can be omitted.

FIG. 8 a is a schematic view of the present invention used in a singlephase input switching power supply with PFC, the control strategy ofwhich is schematically shown in FIG. 3 d. Vpfc is the voltage of the busline capacitor Cpfc, and drv is the voltage of the driving coil of theauxiliary relay K2. Assuming that the electric level of drv is high, theauxiliary relay K2 is actuated, and the auxiliary relay K2 is releasedwhen the electric level of drv is low. The control strategy lies in thatthe main power circuit of the switching power supply module is cut off,and the main relay K1 is released when the input voltage Vin is higherthan the predetermined protection voltage. The charging of the inputpower supply to the power supply module is controlled by controlling theactuation and release of K2, such that the voltage of the bus line Cpfcin the power supply is controlled in certain range, e.g., K2 is turnedON and the input power supply charges Cpfc via Rss when the voltage ofCpfc is lower than Vmin; and K2 is turned OFF when the voltage of Cpfcis higher than the upper limit Vmax. Thus, it is ensured that the busline voltage inside the power supply is controlled within a safe range,to ensure the safety of the devices inside the power supply; and at thesame time to ensure the voltage of the energy supply of the auxiliarypower supply, such that the auxiliary power supply and the controlcircuit can work normally.

At the instance of turning on, cutting off of the aforesaid relays,there may exist voltage and current at the contact point of the relay,thus when the voltage or current of the contact point is relative high,there may be certain damage to the contact point of the relay. Since theinput voltage is a varying sine wave, for input voltage of single phase,Vin=Vpk*sinwt (w=100π). Therefore, there always exist two time periodswhen the absolute value of the input voltage is lower than the outputbus line voltage Vpfc. Thus, as long as the input voltage is lower thanthe bus line voltage at the moment of turning on or cutting off of theauxiliary relay, there is no current at the moment of turning on orcutting off of the relay, namely turning on and cutting off with zerocurrent. FIG. 8B is the waveform of the voltage and current of the zerocurrent actuation of the relay, wherein Vrelay is waveform of thevoltage between the two ends of the relay, Irelay is the current of therelay, and drv is the drive signal of the relay. The relay is actuatedat time t2 when the rectifying bridge is in OFF state due to Vin(t2)<Vmin. Therefore, there is no current in the relay at the time ofturning off. Actually the actuation of the relay has certain time delay,thus the turning on signal drv of the relay needs to be advancedappropriately. As shown in FIG. 8 b, the relay may give the pulse foractuating the coil at time t1 in advance. FIG. 8 c is the releasewaveform of the relay, and zero current switching can be achieved aslong as the input voltage is lower than the bus line voltage, that isVin (t)<Vmax at the release time of the relay. Since the release of thecontact point of the relay has certain time delay, the releasing pulseneeds to be advanced appropriately in practice.

FIG. 9 shows the circuit of FIG. 8 a with a bypass capacitor provided onthe auxiliary relay (namely, the case of embodiment 3). The function ofthe capacitor lies in that the input power supply may continue to chargethe bus line capacitor Cpfc through Rss and C1 when the auxiliary relayis in OFF state, thus the frequency of actuation and release of theauxiliary relay can be reduced. However, the selection of Rss, C1 shouldmeet the following requirement: the charging power of the power supplythrough Rss and C1 is lower than the smallest input power of theauxiliary power supply, and a third impedance Z3 connected in series tothe second electric control switch K2 is required for avoiding arc,wherein the serially connected Z3 and K2 are connected with C1 inparallel.

The present invention can also be used in a circuit without power factorPFC correction, as shown in FIG. 10. The present invention can also beused in a three-phase circuit, e.g., FIG. 11 is an overvoltageprotection circuit of three-phase input without PFC correction, and FIG.12 is the protection mode of a three-phase circuit with PFC.

By only adding simple hardware and control, good effect can be achievedin the present circuit. It is easy to achieve operation under long-termhigh voltage without damage to the module, e.g. it is easy to achieveoperation under long-term high voltage of 415 Vac without damage to themodule in single-phase module by using capacitor of 450 Vdc and powerdevice of 500 V.

1. A switching power supply with overvoltage protection, comprising asoft start circuit (1), a rectifying circuit (2), a filter capacitor(C1), a main power supply (3), an auxiliary power supply (4) and amonitoring circuit (10); the soft start circuit (1) comprising a softstart resistor (Z1) and a first electric control switch (K1) positionedin a branch in parallel with the soft start resistor (Z1); wherein afteran AC current is input into the rectifying circuit (2) for rectificationand into the filter capacitor (C1) for filtering via the soft startcircuit (1), a resulted DC current is output to the main power supply(3) and the auxiliary power supply (4) respectively; the auxiliary powersupply (4) supplying DC voltage to the monitoring circuit (10), whilethe monitoring circuit (10) monitoring an input or/and output voltage ofthe switching power supply, and controlling the first electric controlswitch (K1) in the soft start circuit (1); characterized in that asecond electric control switch (K2) is provided, wherein the secondelectric control switch (K2) is connected in series on a phase line onwhich the soft start resistor (Z1) is provided, the control end of thesecond electric control switch (K2) is connected with the monitoringcircuit (10), the monitoring circuit (10) controlling an ON or OFF stateof the second electric control switch (K2).
 2. The switching powersupply with overvoltage protection according to claim 1, characterizedin that the second electric control switch (K2) and the soft startresistor (Z1) are connected in series, and the serial branch isconnected with the first electric control switch (K1) in parallel. 3.The switching power supply with overvoltage protection according toclaim 1, further comprising a second impedance (Z2) connected inparallel with the second electric switch (K2).
 4. The switching powersupply with overvoltage protection according to claim 1, furthercomprising a second impedance (Z2) and a third impedance (Z3), the thirdimpedance (Z3) and the second electric control switch (K2) beingconnected in series, and then being connected with the second impedance(Z2) in parallel.
 5. The switching power supply with overvoltageprotection according to claim 1, further comprising a second auxiliarypower supply (6), an input end of which is connected with a battery,while an output end of which is connected with the monitoring circuit(10).
 6. The switching power supply with overvoltage protectionaccording to claim 1, characterized in that the first and secondelectric control switches (K1, K2) are relays.
 7. An overvoltageprotection method for a switching power supply, comprising the followingsteps: S1: a monitoring circuit (10) monitors an input voltage or/andoutput voltage of a switching power supply; S2: if AC input is in anallowable range, then ON and OFF states of a first electric controlswitch (K1) of normal open type and a second electric control switch(K2) of normal close type is controlled such that AC charges a filtercapacitor (C1) through a soft start resistor (Z1) and the secondelectric control switch (K2), after completion of a soft start, thefirst electric control switch (K1) is controlled to be turned on and amodule enters a normal operating state; S3: when the AC input is in thesituation of overvoltage, voltage of the filter capacitor (C1) rises, ifthe monitoring circuit (10) detects that the voltage of the filteringcapacitor (C1) is higher than a predetermined voltage threshold, thecontrol circuit (10) cuts off the first and second electric controlswitches K1 and K2, and the energy stored in the filter capacitor (C1)maintains the OFF state of the second electric control switch (K2); S4:when the energy stored in the filter capacitor (C1) is consumed and thevoltage thereof is lowered to a certain threshold, the second electriccontrol switch (K2) returns to the ON state such that AC charges thefilter capacitor (C1) through the soft start resistor (Z1) and thesecond electric control switch (K2), and thereby the voltage of thefiltering capacitor (C1) rises gradually; S5: if the voltage of thefilter capacitor (C1) exceeds the allowable value again, the secondelectric control switch (K2) will be cut off again and the aforesaidsteps S3 and S4 repeat; S6: until the voltage of AC input decreases tobe in a normal range, the states of the first and second electriccontrol switches K1 and K2 are controlled such that the module enters anormal operating state.
 8. The overvoltage protection method for aswitching power supply according to claim 7, characterized in that asecond voltage threshold is set in the monitoring device (10) inadvance, such that in step S4, the second electric control switch (K2)is controlled by the monitoring circuit (10) to return to the ON statewhen it is detected that the voltage is lower than the threshold.
 9. Theovervoltage protection method for a switching power supply according toclaim 7, characterized in that a time of turning on or cutting off ofthe first and second electric control switches (K1, K2) is set such thatthe input voltage at the time is lower than a bus line voltage, and thusthe current is zero when a relay is actuated or released.